Published Swedish patent application 442,232 discloses a cooling-air blower wherein the combustion-air channel lies approximately in a plane parallel to a radial fan wheel and axially delimits a cooling-air spiral surrounding the fan wheel. An annular slit is provided in the combustion-air channel and faces toward the cooling-air channel. The annular slit is covered with respect to the fan wheel by a baffle plate in order to prevent a direct entry of dirt particles entrained by the cooling air into the combustion-air channel. A significant disturbance of the cooling-air flow in the cooling-air spiral occurs because of the long annular slit whereby the cooling of the internal combustion engine can be affected. Furthermore, the annular slit is disposed near the outlet of the cooling-air spiral where there is a relatively high static pressure which facilitates the passage of dirt particles from the cooling-air flow into the combustion-air channel.